This invention relates to the field of preparing accurate gaseous and/or vapor mixtures for analytical instrumentation. Gas mixtures are in widespread use as calibration or reference standards for analytical instrumentation and as feed stock for scaled down chemical reaction or processes particularly in research and development.
Commercial bottled gas and vapor mixtures are available from numerous suppliers for analytical instrumentation use. However, the occasion often arises when it is of advantage to the user to have apparatus capable of mixing gases and/or vapors accurately to predetermined proportions.
Examples of these occasions are as follows:
1. One lacks the time required to obtain a commercially prepared mixture (a minimum of twenty-four hours is generally required although one to two weeks is commonly accepted delivery time);
2. The chemical stability or reactiveness of components in the mixture dictates that it be utilized immediately after preparation;
3. The accuracy of commercial mixtures is in question;
4. Sufficiently accurate mixtures are not easily obtained;
5. The selection of a final mixture requires a "trial and error" procedure by the user;
6. Only small quantities are required; and
7. The components of a mixture tend to stratify (heating and extensive rolling of the vessel are presently the only means for maintaining a homogeneous mixture of components which are easily stratified).
Preparation of gaseous or vapor mixtures by the user is limited to a few devices utilizing mass flow and permeation techniques. These are dynamic devices with blending occurring only when components are flowing. These techniques are not accurate and lend themselves to applications requiring few components in the mixture. Some of the devices can only be applied for specific mixtures. An example of this technique is disclosed in U.S. Pat. No. 3,948,281.
Commercial preparation of gaseous or vapor mixtures falls in two general categories. One is a gravimetric technique in which the vessel and its contents are weighed and the other is a partial pressure technique. The accuracy of the gravimetric method is dependent to a large degree on the weight of each component relative to the total weight of the vessel and its contents. This results in lower accuracies being attained in low density mixtures, such as hydrogen and helium, and also in situations where the components of interest are in low concentration.
The partial pressure method has limited accuracy due to use of high pressures required to make the process commercially feasible, lack of suitable means for homogenizing the mixture, and absence of temperature control. High pressures produce large compressibility factors which are not predictable with any degree of accuracy, being dependent on the composition and state of intermediate and final mixtures.
Lack of homogenizing capability and temperature control results in large errors due to variations in temperature during the vessel filling process which are caused by decompression and compression of gases. The partial pressure technique, as has been practiced commercially and by end users, is not a suitable method for the preparation of accurate gas or vapor mixtures. It is best used for preparing "target" concentrations, followed by analysis, or for blending low grades of calibration gases. Reference is made to the 1975 copyrighted book Gas Mixtures -- Facts and Fables, by Frank Scarporoicer that is available from Matheson Gas Products Company, 932 Paterson Plank Road, P.O. Box 85, East Rutherford, New Jersey 07073 and which is hereby incorporated by reference for all purposes herein.
Very low concentrations of gaseous or vapor mixtures are also difficult to prepare accurately and often require elaborate procedures, some of which are suitable only for specific compounds. An example is gas permeation, which is a dynamic technique, requires precision temperature control, has a limit on active component life, and has a narrow range of applicability.
In general, both high and low gas or vapor mixtures have previously required both a preparation procedure followed by an analysis procedure to confirm composition of the mixture.
Widespread use of analytical instruments such as gas chromatography apparatus now exists in the chemical process industry as well as in other fields. For example, see U.S. Pat. No. 3,595,063 to Loew as well as U.S. Pat. No. 3,888,109 to Sharki for a support system increasing the capability of a gas chromatograph. Such gas chromatographs are capable of rapidly analysing and indicating the presence of the mixture components in a gas sample both qualitatively and quantitatively. Since gas chromatographs enables a process operator to quickly determine the composition of a sample, more frequent analyses of the various process stream are available for enhancing process efficiency.
One major drawback to the many advantages of the use of gas chromatographs apparatus has been the need for a reference or calibration sample by which the gas chromatograph apparatus is both calibrated and from time to time tested for accuracy. Such reference samples have previously been prepared by chemists working in the laboratory using the techniques set forth above.
Gas blenders are known as evidenced by U.S. Pat. No. 2,950,618 to Lewis. Furthermore, systems have been developed for determining the concentration of a gas in a mixture, such as disclosed in U.S. Pat. No. 2,817,350 to Bradner, et al. Also, as disclosed in U.S. Pat. No. 3,817,085, polarographic sensors or electrolytic sensors may be used to measure the partial pressure of a component in a mixture.